The present invention relates to a dry etching method for use in the field of microfabrication and, more particularly, to a dry etching method for high-precision microfabrication of a miniature structure of the submicron order with a high aspect ratio and with low surface roughness. The invention also pertains to a photonic crystal device fabricated by use of such a dry etching method.
In the field of microfabrication for integrated circuits and MEMS (Micro-Electro-Mechanical-System) devices there is a growing need for the technique to microfabricate a structure of the submicron order with a high aspect ratio. Furthermore, it is pointed out in the art that the surface roughness of the microfabricated device structure, for example, the surface roughness of its bottom and sidewalls determines the device performance; hence, there is an earnest desire for reduction of the surface roughness. At present, it is customary in the art to fabricate such microminiature devices by use of such dry etching methods as plasma etching using reactive plasma and ion milling that irradiates a specimen or workpiece with a monomer ion such as Ar+. In particular, the reactive plasma etching is excellent in performance and in cost-performance, and for this reason it is currently the mainstream of microfabrication.
With a view to minimizing damage to the specimen, there is proposed an etching method using a gas cluster ion beam. This is a method that irradiates the specimen with ionized clusters formed by the agglomeration of gas molecules, and it is known that this method permits minimization of damage to the specimen since the charge or energy of the ionized clusters is very small in terms of incident atoms. For example, in the case of accelerating an Ar+ monomer ion and an Ar cluster ion composed of 1000 atoms with a 1-kV voltage, the kinetic energy per atom in the irradiation of a specimen substrate with the Ar cluster ion is as low as 1 eV, so that it is harder for the Ar cluster ion to penetrate into and hence cause damage to the specimen than in the case of the Ar+ monomer ion. Besides, since the amount of charge is small as compared with the number of atoms that contribute to etching, the irradiation with the Ar cluster ion achieves a high etching ratio while getting around such problems as charging of the substrate and dielectric breakdown of an element containing an insulating structure.
Prior art examples that use such cluster ion beam for etching are disclosed in Japanese Patent Kokai publications Nos. H03-163825 (published Jul. 15, 1991, hereinafter referred to as Document 1) and H05-102083 (published Apr. 23, 1993, hereinafter referred to as Document 2). In Document 1 there is set forth an example that uses cluster ion beam etching for the purpose of minimizing damage to the surface of a specimen (a silicon substrate) by irradiation with the beam and performs etching the silicon substrate by irradiating it with the Ar cluster ion beam in a Cl2 gas atmosphere. In Document 2 there is disclosed an example using the cluster ion beam etching as a solution to the problems attendant with the plasma etching, such as irradiation damage to the specimen surface and etching profile abnormalities caused by the deflection of the incident ion beam due to charging of the specimen. In Document 2 it is pointed out that when a silicon substrate having its surface covered with a mask as of SiO2 was etched by Cl2 gas, there was not observed a tapered etching profile considered to be caused by charging of the substrate.
In plasma etching, there is known a technique called “sidewall protection” that permits vertical etching even if the incident angle of the ion beam to the specimen surface is distributed (e.g., Hajime Tokuyama, “Semiconductor Dry Etching Techniques,” Sangyoh Tosho, pp. 64-66, Oct. 6, 1992, hereinafter referred to as Document 3). For example, in the case of etching a silicon substrate by the plasma etching scheme using the Cl2 gas alone, sidewalls of the groove are hollowed out by some components of the plasma ion which are incident on the silicon substrate at some angles thereto, but the addition of CHF3 to the Cl2 implements vertical etching without hollowing out the sidewalls of the groove. This is explained to be due to the formation of CHF3-polymerized films on the sidewalls which resist etching by the diagonally incident ions. Thus, in the field of plasma etching it is well-known that vertical etching can be achieved by the combined use of the etching gas and the gas capable of forming the sidewall protective film. An example of this sidewall protection is also disclosed in Japanese Patent Kokai Publication No. H06-349784 (published Dec. 22, 1994, hereinafter referred to as Document 4).
In Japanese Patent Kokai Publication No. H10-135192 (published May 22, 1998, hereinafter referred to as Document 5) there is disclosed a plasma etching method in which an etching gas as of SF6 and a passive gas for forming the sidewall protective film, such as CHF3, are not mixed but instead they are used alternately with each other. This is a method generally called Bosch process, which provides excellent verticality of sidewalls and ensures implementation of a high aspect ratio. Furthermore, in Japanese Patent Application Kokai Publication No. H09-082691 (published Mar. 28, 1997, hereinafter referred to as Document 6) there is disclosed another conventional method which performs etching while at the same time protecting sidewalls. With this method, during etching at least one of the product resulting from the reaction between a material being etched and a halogen-based gas and a re-dissociation product of the reaction product is deposited on the material subject to etching. This method permits efficient deposition of a protective film on the sidewall near the part being etched.
In recent years, a method for planarizing or flattening the workpiece surface by use of a gas cluster ion beam has come to industry attention because it reduces damage to the workpiece surface and the surface roughness. For example, in Japanese Patent Kokai Publication No. H08-120470 (published May 14, 1996, hereinafter referred to as Document 7) there is described a method for reducing the surface roughness by irradiating the workpiece surface with the gas cluster ion beam. With this method, the gas cluster ion applied to the workpiece is broken up into particles by its collision with the workpiece, and at this time a multibody collision occurs between cluster forming atoms or molecules and workpiece forming atoms or molecules, by which the particles are drive hard in a direction parallel to the workpiece surface—this permits cutting in a direction lateral or parallel to the workpiece surface. This is a phenomenon called “lateral sputtering,” in which surface protrusions or convexities are cut away by movements of the particles across the workpiece surface to achieve its ultraprecision polishing on the order of atomic size. Since the energy of the ion of the gas cluster ion beam is far lower than in ordinary ion etching, such ultraprecision polishing can be done without damaging the workpiece surface.
In the surface planarization by use of the gas cluster ion beam, it is generally recognized that the gas cluster ion beam may preferably be incident on the workpiece substantially at right angles to the surface thereof. This is intended to make utmost use of the effect of surface smoothing by the above-mentioned “lateral sputtering.”
With the method that provides sidewall protection during etching, it is possible to achieve vertical etching as referred to above, but the use of an ordinary ion beam allows the workpiece surface to become rough, and by the mechanism for coating the etching profile over the entire area of its surface with the protective film, the etching profile tends to taper off as the etching proceeds. In FIG. 1, steps S1 to S5 depict how the etching profile tapers off. In FIG. 1, reference numeral 11 denotes a substrate, 12 a mask, 13 a groove, and 14 a sidewall protective film. Such a taper-off tendency does not matter when the aspect ratio of the etching profile is small as shown in FIG. 2A, but it gives rise to a serious problem when the aspect ratio is large as shown in FIG. 2B, and in the fabrication of devices which calls for very high-precision etching, such as a photonic crystal device and the like.
With the Bosch process which alternately performs etching and deposition of the sidewall protective film, the etching scheme is designed so that the sidewall 15 of the groove 13 just underlying the marginal edge of the mask 12 is also etched away as depicted in steps S1 to S4 in FIG. 3, by which it is possible to prevent the groove 13 from tapering off; however, a structure called “scallop” is formed on the sidewall 15 of the groove 13 as shown in step S4 in FIG. 3—this produces the serious problem of increased surface roughness of the sidewall 15.
For the reasons given above, such a plasma etching method combined with the “sidewall protection” scheme cannot be used to manufacture devices which require a large aspect ratio, high precision and low roughness, such as photonic crystal devices required to have an average surface roughness Ra on the order of 0.1 nm.
On the other hand, Documents 7, 1 and 2 describe the use of the cluster ion beam that enables planar ultraprecision polishing of the workpiece surface in terms of atomic size and the use of the cluster ion beam for etching that permits reduction of damage to the workpiece surface, as referred to previously, but these documents indicate only the reducibility of the surface roughness of the workpiece by use of the cluster ion beam and make no mention of the workability of vertical etching and the sidewall surface roughness. In other words, no studies have been made of an etching method for microfabrication of devices with a large aspect ratio, high precision and low roughness.